Abstract
So far, significant studies have been performed to increase the shear strength (τ) of ballast by reinforcing it with geogrid, though ballast encasement with geogrid, which could provide substantial confining pressure, has not yet been considered. Therefore, in this study, using a large-scale direct shear apparatus, effect of encasement of ballast with geogrid on shear behavior were investigated. For this purpose, two groups of ballast with average particle size (D50); 41.31 mm and 31.55 mm, and three types of biaxial geogrids GP35/35, GP40/20, and GP60/20, were considered. These two sets of experiments were carried out under normal stresses (σn) of 50, 100, and 150 kPa and with a shear rate of 1 mm/min. Experimental results showed that in the non-encased case, τ was affected by nominal maximum particle size (Dmax), uniformity coefficient (Cu) as well as the applied σn. According to the results σn was increased from 50 to 150 kPa, the internal friction angle (φ) for group 1 decreased from 65.56° to 54.39° and for group 4 it decreased from 63.90° to 52.16°. Similarly, for the above-mentioned gradation groups, the dilation angle (ψ) decreased by 16.69% and 12.85% on average for all σn s, respectively. On the other hand, ballast encasement with geogrid improved its shear behavior, as for gradation group 1, the maximum effect of encasing on the φ corresponding to σn alteration from 50 to150 kPa was 74.5° for group 1 and 68.96° for group 4. While, the maximum reduction in ψ was 32.22% on average under these conditions, moreover for ballast group 4, the mentioned changes were 30.13%, 14.67%, and 27.86%, respectively. Also, in this study, the efficiency factor (α), varied in the range of 1.11–1.91. The effect of geogrid aperture size (A) on τ of encased ballast was investigated, too.
Published Version
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